The swift success of an al Qaeda-inspired insurgency in Iraq over the past week has led to heightened concerns over deepening U.S. involvement in a conflict that most Americans had thought was behind them.

Last week saw the capture of Iraq’s second-largest city, Mosul. The black-clad Sunni militants from the Islamic State of Iraq and al-Sham, also known as the ISIS or ISIL, introduced strict religious law on the city, barring religious freedom and requiring women wear to veils. The radical group later advanced south, seizing Saddam Hussein’s hometown of Tikrit.

As the turmoil continued Sunday with a series of explosions in Baghdad, new questions are being raised — can the assault by the Sunni militants can be stopped, will the U.S. military bolster its presence in Baghdad and how will the instability in Iraq affect the rest of the Middle East.

How involved is the U.S.?

On Sunday, the USS George H.W. Bush and two other U.S. Navy ships arrived in the Persian Gulf. The move came after President Barack Obama told reporters last week that his national security team was "looking at all the options" in Iraq and that he wouldn't "rule out anything."

The U.S. Embassy in Baghdad remained open in the aftermath of the explosions, though security was strengthened — including the addition of "fewer than 100" Department of Defense personnel, according to a military official.

The State Department said in a statement that some staff members were transferred out of the city.

"The people of Iraq have repeatedly rejected violent extremism and expressed their desire to build a better society for themselves for their children," the statement said.

The State Department also issued a travel warning Sunday, noting that "the ability of the Embassy to respond to situations in which U.S. citizens face difficulty, including arrests, is extremely limited....U.S. citizens in Iraq remain at high risk for kidnapping and terrorist violence."

As the violence has escalated, Republican lawmakers sharply criticized Obama's push to withdraw American troops in Iraq and argued for a more robust use of the U.S. military, including the use of air strikes. Democratic Senator Joe Manchin of West Virginia told NBC's David Gregory on Sunday that if the president got "his intelligence group back on track," he would be "open" to air strikes, too.

But on Sunday, a wave of attacks targeted downtown Baghdad and the Sadr City district. By Sunday afternoon, the streets were empty and the city was under a nighttime curfew.

On Sunday, several graphic photos connected to the militants surfaced that purportedly showed them killing Iraqi soldiers en masse. The photos have not been verified by NBC News. but show captive men being loaded onto trucks and made to lie down in a ditch. The last images in the series show bodies soaked in blood.

This image posted on a militant website on Saturday, which was provided by AP and has not been verified by NBC News, appears to show militants from the al-Qaeda-inspired Islamic State of Iraq and al-Sham leading captured Iraqi soldiers to an open field in Tikrit, Iraq, moments before shooting them.Militant website via AP

In a statement, a spokeswoman for the State Department said that the reports of the mass murders could not be confirmed, but spokeswoman Jen Psaki, added, "The claim by the Islamic State of Iraq and the Levant (ISIL) that it has massacred 1700 Iraqi Shia air force recruits in Tikrit is horrifying and a true depiction of the bloodlust that these terrorists represent."

What are other countries saying?

In an appearance on state-run television on Saturday, Iranian President Hassan Rouhani offered to help solve the crisis in Iraq — as long as Washington agrees to "fight terrorist groups in Iraq in elsewhere," he said. "We all should practically and verbally confront terrorist groups," Rouhani said.

On Sunday, Republican Senator Lindsey Graham conceded that for Baghdad not to fall, the U.S. may have to work with Iran. "We are probably going to need their help to hold Baghdad," Graham said on CBS' "Face the Nation."

He added: "The Iranians have an interest. They have Shia populations to protect. We need a dialogue of some kind."

Former British Prime Minister Tony Blair told Sky News on Sunday that the West will have to grapple with turmoil in Iraq. "I'm not suggesting we put ground troops in and we do a full scale invasion as we did in Iraq or Afghanistan, but I am saying we are going to have to take an active role in trying to shape events in Syria and Iraq and indeed across the region," he said.

Secretary of State John Kerry on Saturday assured the Iraqi foreign minister that the United States was, "reaching out to the international community and Iraq's regional neighbors to emphasize the threat that Iraq and the region are under from ISIL and the importance of coming to Iraq's aid at this critical juncture," according to a statement.